Electronics > Metrology

Resistor values and tolerance for checking multimeters

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001:
Hi! My stupid question

At my homebrew "lab" I have few used meters:
Instek GDM-8245 50000 counts ± (0.1% + 4) for 500 Ohm range and ± (0.1% + 2) for others
Fluke 87V  6000 counts ± (0.2 % + 1) for 600k range and  ± (0.6 % + 1) for all above
Fluke 117  6000 counts ± (0.9 % + 2) for 600 Ohm range and  ± (0.9 % + 1) for all above

Can I "check" all of them with NOS wirewound 0.25% 1ppm 1/2W resistors ? (Since 0.25% for 0-300k resistor values is lower than 0.2% for 600k multimeter RANGE for example)
What values I must use? 1/2 of range (I mean 300Ohm/3k/30k/300k for 6000 Count meter) or similar to range nominal (I mean 560Ohm/5k6/56k/560k for 6000 Count meter)?

Edwin G. Pettis:
The normal minimum accuracy ratio for calibration is 5:1, preferential is 10:1, however if your requirements are not too high on absolute accuracy you can get away with a bit lower ratio, say 3:1 or 4:1, anything less than that will not provide enough accuracy to get your meters close to stated accuracy.  Since the best accuracy is ±0.1%, ±.025% or ±.02% resistors would work there, TCR <5 PPM/°C.  In the case of these meters, mid-range values should suffice to check accuracy.  You can certainly use resistors close to the full range value but make sure they are slightly lower than full scale.

Cerebus:
That's a 'me too' to everything Edwin said. Those measurement ratios he's talking about are called "test uncertainty ratios" (TUR) by the metrology crowd; I mention it as it might be a useful search term for you.


--- Quote from: 001 on November 01, 2017, 06:14:35 pm ---Can I "check" all of them with NOS wirewound 0.25% 1ppm 1/2W resistors ? (Since 0.25% for 0-300k resistor values is lower than 0.2% for 600k multimeter RANGE for example)

--- End quote ---

I think from the way you've put that, that perhaps there is a slight misunderstanding of how those accuracy figures are normally presented and what they mean.

In figures like "± (0.6 % + 1)" the first figure is for the uncertainty in the reading, the second the uncertainty in the range. You may also see figures such as "± (0.6 % + 0.06%)" or "± (200ppm + 100ppm)". You can read "± (0.6 % + 0.06%)" as "± (0.6% of reading + 0.06% of range)". They typically represent the effects of gain (and linearity) errors and offset errors respectively.

Let's have a concrete example, drawn from the figures you've presented. Let's pick


--- Quote from: 001 on November 01, 2017, 06:14:35 pm ---Fluke 87V  6000 counts ± (0.2 % + 1) for 600k range and  ± (0.6 % + 1) for all above

--- End quote ---

and a resistor under test of 300k. Let's assume that the resistor is perfect, an exact 300k. On the 600k range the error would be "± (0.2 % + 1 count)" with 6000 counts representing 600k so 1 count 100 ohms, so that calculates out as "± (600 ohms + 100 ohms)" or simply "± 700 ohms". So with that 'perfect' 300k resistor our meter could read anything from 299,300 to 300,700 ohms.

To meet a 10:1 TUR for that range with a 300k resistor you'd need a resistor whose true value is 300k ±70 ohms, or 300k ± 0.023%. A marginal 4:1 TUR would call for a 300k ± 0.0583% resistor.

Note by the way, as Edwin hints, some meters have calibration routines that rely on testing against a 'near full range' resistor. My Keithley 197 specifies a 190 ohm calibration resistor for the 200 ohm range, and so on.

001:
Thank You!
Awesome answers  :-+

Can You talk me also: if I use TWO serial soldered resistors what is resultated tolerance?
For example 10k[± 0.5 %] + 100k[± 0.25 %] = 110k[± ?? %]

And what about overall tolerance if I solder it parallel? 9.(09)k[± ?? %]

e61_phil:

--- Quote from: 001 on November 02, 2017, 01:06:41 pm ---Thank You!
Awesome answers  :-+

Can You talk me also: if I use TWO serial soldered resistors what is resultated tolerance?
For example 10k[± 0.5 %] + 100k[± 0.25 %] = 110k[± ?? %]

And what about overall tolerance if I solder it parallel? 9.(09)k[± ?? %]

--- End quote ---

You can simply caculate the worst case error of such a circuit.

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